The present invention relates to a method of polishing a copper layer of a substrate, e.g., a build-up substrate, used for example as a semiconductor wafer.
Some multi-layered circuit boards, on which electronic parts, e.g., semiconductor chips, etc. will be mounted, are made by a build-up method.
The build-up method will be explained.
Firstly, as shown in FIG. 9, an insulating resin layer 12 is formed on lower cable patterns 11, and then via-holes 13 are formed so as to expose a part of each lower cable pattern 11.
Next, as shown in FIG. 10, a copper layer 14 is formed in the via-holes 13 and a surface of the insulating resin layer 12 by non-electrolytic plating and electrolytic plating. In some cases, the copper layer 14 fills an inner space of each via-hole 13. The copper layer 14 formed by plating is apt to be thick in the vicinity of edges of the insulating resin layer 12. Therefore, parts of the copper layer 14 corresponding to the via-holes 13 are projected upward, so that flatness of the surface of the copper layer 14 must be low.
To make the surface of the copper layer 14 flat, firstly the surface of the copper layer 14 is removed by a roll-buff. Then, the copper layer 14 is etched to form into prescribed cable patterns (upper cable patterns). With this method, the upper cable patterns are electrically connected to the lower cable patterns 11. This method will be repeated prescribed times to form the multi-layered circuit board.
In the case of forming circuits on a semiconductor wafer, connecting terminals, e.g., copper bumps, are formed in the wafer after the circuits are formed therein. The copper bumps are formed by the steps of: forming a resist mask having opening sections, which correspond to the copper bumps, on the wafer; plating parts of circuit sections exposed in the opening sections with copper; and removing the resist mask. With this method, the copper bumps can be projected from the surface of the wafer. Note that, the wafer having the copper bumps will be cut and divided into a plurality of semiconductor chips. Each of the semiconductor chips will be electrically connected to a circuit board by the copper bumps as terminals.
In some cases, copper bumps are formed in a circuit board so as to electrically connect a semiconductor chip(s) to the circuit board.
However, the above described conventional methods have following disadvantages.
The roll-buff is made of hard abrasive grains, which are bound by binders and formed into a cylindrical shape. The roll-buff is rolled on the surface of the copper layer so as to remove the projected parts of the copper layer. The projections can be removed by the roll-buff, but waviness in a wide area cannot be removed by the roll-buff, so that it is difficult to form the copper layer having uniform thickness. Further, the surface of the copper layer is scratched by the roll-buff, so that reliability of an electric element must be lower.
On the other hand, height of the copper bumps must be equal so as to securely connect the copper bumps. It is desirable to remove the copper bumps to make their height equal, but there are no effective methods.
The present invention has been invented to solve the problems of the conventional methods.
An object of the present invention is to provide a method of polishing a copper layer of a substrate, which is capable of improving a stock removal rate, forming a copper layer having uniform thickness, restricting scratches on a surface of the copper layer, and forming copper bumps having equal height.
Namely, the method of the present invention comprises the steps of:
supplying a substrate having a copper layer onto a polishing pad on a polishing plate with the copper layer facing the polishing pad;
pressing the substrate onto the polishing pad, with a backing pad, by a press head;
relatively rotating the press head with respect to the polishing plate, with supplying polishing slurry onto the polishing pad, so as to polish the copper layer,
wherein the backing pad is made of a material whose Asker C hardness is 75-95 and whose compressibility is 10% or less,
and the polishing slurry includes a chelating agent for chelating copper, an etching agent for etching the surface of copper layer, an oxidizing agent for oxidizing the surface of copper layer, and water.
Copper bumps formed in the substrate can be effectively removed by the invention. Height of the copper bumps can be made equal, and the surface of the substrate can be uniformly polished.
Further, a copper layer formed in a build-up substrate can also be effectively polished by the invention.
In the method of the present invention, a hard and low compressible backing pad, whose Asker C hardness is 75-95 and compressibility is 10% or less, is used.
Therefore, the substrate is held by the press head and the backing pad contacts a back side of the substrate when the copper layer (a front side) of the substrate is polished.
In this case, if the backing pad is too soft, a pressing force applied to projections in the front side or a counter force from the polishing pad is transmitted to the backing pad and the backing pad is depressed by the projections. Namely, the parts of the substrate, which correspond to the projections, are buried in the backing pad, so that it is difficult to remove the projections in the front side of the substrate and the copper layer cannot be uniformly polished.
On the other hand, if the backing pad is too hard, e.g., Asker C hardness is 96-100, the back side of the substrate becomes a standard face for the polishing. If there are small projections in the back side of the substrate, the projections are not buried in the backing pad, so that the projections badly influence the polishing. Uniformity of the polished copper layer must consequently be lower.
In the method of the present invention, the backing pad has the prescribed hardness and compressibility, so that projections in the back side of the substrate can be removed by the backing pad, and the backing pad is not badly influenced by projections in the copper layer. Therefore, the copper layer can be polished uniformly, and the copper layer having uniform thickness can be produced.
In the method, the backing pad may be made of polyurethane foam.
Further, the polishing slurry used in the method is capable of increasing the stock removal rate without scratching the surface of the copper layer, so that manufacturing efficiency can be improved. Namely, the polishing slurry includes a chelating agent for chelating copper, an etching agent for etching the surface of the copper layer,
an oxidizing agent for oxidizing the surface of the copper layer, and water. The copper layer is chemically etched by the etching agent.
The chelating agent catches copper grains, which are removed and separated by the etching agent and the abrasive grains in the case of the polishing slurry includes abrasive grains, and acts as the accelerator of the polishing.
Further, the chelating agent prevents the surface of the copper layer from being scratched by copper grains, so that no surface defects are formed on the surface of the copper layer.
In the method, the chelating agent may be organic carboxylic acid. For example, it may be at least one selected from the group consisting of: amino acid; quinoline-2-carboxylic acid (quinaldic acid); 2-pyridine carboxylic acid; 6-pyridine carboxylic acid; and quinine. Preferably, amino acid is selected due to good environmental conditions and high stock removal rate. Further, the chelating agent may be at least one amino acid selected from the group consisting of: neutral amino acid, such as glycine, xcex1-alanine, -alanine, valine, leucine, L-isoleucine, D-isoleucine, L-alloisoleucine, D-alloisoleucine, serine, L-threonine, D-threonine, L-allothreonine, D-allothreonine, cysteine, methionine, phenylalanine, tryptophan, tyrosine, proline, cystine; and basic amino acid, such as lysine, arginine, histidine. Especially, the preferable chelating agent is at least one selected from the group consisting of: glycine, xcex1-alanine and -alanine due to high stock removal rate.
Amount of the chelating agent in the slurry should be 0.015-2.5 mol/l. If it is less than 0.015 mol/l, the stock removal rate must be low; if it is more than 2.5 mol/l, some chelating agents are deposited. Attention is highly required.
Preferable amount of the chelating agent in the slurry is 0.03-2.0 mol/l; further preferably, 0.05-1.8 mol/l.
The etching agent chemically etches the surface of the copper layer. It further accelerates the mechanical polishing, in which the slurry including abrasive grains is used.
Whatever materials which have an etching force against copper may be used.
Copper can be etched by an acid etching agent and an alkaline etching agent. Preferably, the etching agent is at least one selected from the group consisting of: ammonia; and ammonium salt due to synergism with the oxidizing agent. For example, the ammonium salt is at least one selected from the group consisting of: inorganic ammonium salts, e.g., ammonium carbonate, ammonium hydrogen carbonate, ammonium phosphate, ammonium nitrate, ammonium sulfate, ammonium chloride; and organic ammonium salts, e.g., ammonium lactate, ammonium citrate, ammonium malate, ammonium oxalate. Preferably, ammonium carbonate is selected due to good environmental conditions and high stock removal rate.
Amount of the etching agent should be 5-25 wt % of the polishing slurry. If it is less than 5 wt %, the stock removal rate is too low; if it is more than 25 wt %, some etching agents are deposited. Attention is highly required.
Preferable amount of the etching agent of the polishing slurry is 10-25 wt %, further preferably, 15-25 wt %.
The etching agent may be added with high density, and it may be diluted when it is used. In the case of using ammonia (ammonia water) as the etching agent, preferably it is added immediately before the use due to stability of the polishing slurry.
The oxidizing agent chemically oxidizes the surface of the copper layer. The chelating agent works to the oxidized copper layer, and in the case of the polishing slurry includes abrasive grains, the abrasive grains mechanically works thereto, so that the polishing is further accelerated. Namely, the oxidizing agent accelerates the polishing with the abrasive grains.
Note that, the preferable oxidizing agent is hydrogen peroxide water due to oxidizing force and cost.
Amount of the oxidizing agent should be 0.1-10 wt % of the slurry. If it is less than 0.1 wt %, the stock removal rate is too low; if it is more than 10 wt %, a container for storing the polishing slurry is expanded and stability of the polishing slurry is low.
Preferable amount of the oxidizing agent of the polishing slurry is 0.5-8 wt %, further preferably, 1-5 wt %.
The oxidizing agent of high density may be prepared, and it may be diluted with a solvent when it is used. Preferably, the oxidizing agent is added immediately before the use due to stability of the polishing slurry and prevention of natural dissolution.
The water is used for dispersing and dissolving the agents. Preferably, the water is ion-exchanged water, which has been filtered to remove impurities, or distilled water.
A pH value of the polishing slurry should be 7-10. If it is less than 7, the stock removal rate is too low; if it is more than 10, the slurry is formed into gel and the surface of the copper layer is made rough.
In the method of the present invention, the polishing slurry may include abrasive grains which are made of at least one selected from the group consisting of: silicon dioxide; aluminum oxide; cerium oxide; titanium oxide; silicon nitride; and zirconium oxide, and amount of the abrasive grains may be 0.1-50 wt % of the polishing slurry.
In the case of the polishing slurry includes abrasive grains, the copper layer can be mechanically removed, too. Surface roughness of the polished copper layer can be improved, and further precise polishing can be executed. By the mechanical polishing, the amount of the etching agent can be reduced, so that influences of the chemical polishing can be reduced. Therefore, the surface roughness of the polished copper layer can be improved.
If the amount of the abrasive grain is less than 0.1 wt %, the surface of the copper layer is made rough; if it is more than 50 wt %, mixture and dissolution of other agents are difficult. Preferable amount of the abrasive grains is 0.5-40 wt %, more preferably, 1-35 wt %.
Note that, a preferable material of the abrasive grains is silicon dioxide, and more preferably, colloidal silica.
Average diameter of the abrasive grains will be explained. In the case of the abrasive grains made of silicon dioxide, the preferable average diameter is 10-300 nm (measured by BET method), more preferably 30-100 nm. In the case of the abrasive grains made of cerium oxide, the preferable average diameter is 0.01-0.5 xcexcm (observed by a scanning electron microscope), more preferably 0.05-0.45 xcexcm. In the case of the abrasive grains made of other materials, the preferable average diameter is 0.01-2 xcexcm (measured by a laser diffraction particle size analyzer LS-230 manufactured by Coulter Inc., USA), more preferably 0.05-1.5 xcexcm.
As described above, undiluted solutions of said agents may be prepared with high density, and they may be mixed with proper rates and properly diluted by the water. In this manner, they can be easily and effectively stored and transported. Further, a high density mixture, which includes the chelating agent, water and the abrasive grains (in the case of the polishing slurry includes abrasive grains), may be previously prepared, and the etching agent (in the case of using ammonia water) and the oxidizing agent may be added, with prescribed rate, to the mixture, then the mixture may be further diluted with water so as to use as the polishing slurry.